Gas-liquid separators are commonly used to segregate a gas phase from a liquid phase in many industries, including the oil and gas industry. Effective separation is necessary to ensure the quality of the final products and to prevent problems in downstream process equipment. Many gas-liquid separators employ a mist extractor in the gas phase area of the separator to enhance the coalescence and capture of small liquid droplets entrained in the gas. Mist extractors work on one or more principles including, but not limited to, gravity, enhanced gravity, inertia, and impingement.
At low and medium operating pressures, the gas phase area of conventional gas-liquid separators with mist extractors is typically sized using the Souders-Brown equation:vG(MAX)=K((ρL−ρG)/ρG)1/2 where vG(MAX) is the maximum design gas velocity through the mist extractor in meters per second, K is an empirically determined sizing factor in meters per second, ρL, is the density of the liquid in kilograms per cubic meters, and ρG is the density of the gas in kilograms per cubic meters. Once vG(MAX) has been determined, the following equation may be used to determine the cross-sectional area of the separator:Area (m2)=gas flow rate (m3/s)/vG(MAX) 
Additional equipment may be installed upstream or downstream of the mist extractors to enhance droplet removal. For example, a coalescing element known as an agglomerator may be installed upstream of the mist extractor. Agglomerators are typically mesh pads and act by coalescing fine droplets into larger ones. However, they cannot capture the droplets. Instead, the velocity of the gas stream blows the enlarged liquid droplets from the agglomerator into the mist extractor, where they are captured. As another example, a wire mesh pad may be installed downstream of a more open-type mist extractor, such as a vane, as a polishing element. The polishing element will typically have the same, or larger, cross-sectional flow area as the mist extractor and is installed solely to improve mist extraction. The polishing element is generally of a different type than the mist extractor.
All mist extractors are capacity-limited. If gas flow exceeds the maximum allowable velocity, re-entrainment of the coalesced liquid droplets into the flowing gas stream will occur or pressure drop will become excessive. If gas flow falls below the minimum allowable velocity, there may not be enough energy in the gas stream to bring about the desired coalescence and capture of liquid droplets. As a result, mist extractors function most effectively within an operating range between these maximum and minimum velocities. The ratio of the maximum allowable velocity to the minimum allowable velocity is defined as the turndown of the mist extractor. For example, if the mist extractor has a turndown of 3:1, the minimum design operating flow rate is one third of the design maximum flow rate. Similarly, the K-factor associated with the minimum design operating velocity is one third of the design maximum K-factor.
Because of mist extractor turndown, gas-liquid separators are designed to operate within a specific turndown, defined as the ratio of the maximum gas flow rate to the minimum gas flow rate. Some gas-liquid separator designs may be less affected by flows that are too low because low flow rates increase the gas phase retention time, allowing some additional separation. As a result, the turndown of the separator may be greater than the turndown of the mist extractor. However, this is generally not the case for gas scrubbers which have very little retention time or for separators in critical service. The design limit for these separators is set by the turndown of the mist extractor.
Gas flow rates may vary widely, particularly in oil and gas production separators. These variations in flow often exceed the turndown of the mist extractor, causing the separator to operate below its design flow range some of the time. Therefore, a need exists to extend the turndown of gas-liquid separators. Extending this turndown increases the time the separator operates within its design flow range and can have practical advantages where flow is surging. In those cases, gas flow may drop off during intervals. Extending the turndown allows for good de-misting during those drop-off periods.